The
Midas epithermal system is one of the larger low-sulfidation epithermal vein
systems discovered in the western US in recent years, containing in excess of 3
Moz of gold (premining). The deposit is spatially and temporally related to
bimodal volcanism in the Northern Nevada Rift; dates on igneous rocks (16 - 14.7
Ma) bracket the age of high grade veins (15.3 Ma). Hydrothermal alteration at
Midas is typical of epithermal systems, comprising proximal potassic through
distal propylitic assemblages; in contrast to many epithermal systems, Midas has
minimal development of wallrock illite/sericite. Mineral assemblages are clearly
zoned on the vein structures that acted as focal planes for fluid upflow.
Geochemical zoning patterns also show this zoning. Metals were deposited in
open-space veins as the result of boiling of hydrothermal fluids; multiple
episodes of crustiform mineral deposition are documented in the vein textures.
The major vein minerals include quartz, adularia, calcite, selenides and
sulfides, with electrum as the primary ore mineral. REE patterns for vein
material are similar to patterns for coeval rhyolites, but distinctly different
from patterns of the host rocks. Stable isotope data are consistent with
meteoric water as the dominant ore fluid. Combined REE and stable isotope data
suggest that fluid from which the main ore-bearing stage of the veins was
deposited was slightly lower pH than other stages.